1. Field of the Invention
The present invention relates to a radiation source, lithographic apparatus, and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (resist). Instead of a mask, the patterning means may comprise an array of individually controllable elements that generate the circuit pattern on an impinging light beam.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one pass, and scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”—direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
The size of features that can be imaged by a lithographic projection apparatus is limited by the wavelength of the radiation used. To image smaller features requires a shorter wavelength and so UV, deep UV (DUV) or extreme UV (EUV) radiation is used. The wavelengths that can be used are limited by the sources available, for example, HG vapor lamps for UV, excimer lasers for DUV, and plasma discharge sources for EUV. Semiconductor light sources, such as LEDs and laser diodes, are extremely efficient light sources that are in widespread use in many fields, but as yet there is no such device useful to provide exposure radiation for lithography.
The wavelength of the light output by most current LEDs is determined by the specific semiconductor band structure in the pn-junction. The application of an electrical supply to the junction causes electrons to be injected into the n-type region where they occupy the conduction band of this region (e.g., higher energy bands), as the valence band is full. As the occupancy of this band increases electrons will also be pushed into the conduction band of the p-type region. However, the valence band of the p-type region has some vacancies, so electrons will fall into these lower energy states, emitting light of frequency characteristic of the band-gap in the p-type region in order to conserve energy.
Another type of device produces radiation from a pn-junction using the avalanche effect. The avalanche effect is a process whereby a reverse bias is applied to a suitably doped pn-junction diode. Electrons tunnel across the forbidden depletion region and multiply rapidly, unless strictly controlled. The usual result is thermal emission in the infra-red region. Further information regarding such physical processes can be obtained from: www.tpub.com/neets/book7/26.htm, which is incorporated by reference herein in its entirety. Relevant information can also be found in “Light Emission In Silicon in the Visible Range From Nanoscale Diode Anti-fuses” by V E Houtsma et al., Proceedings from the 3rd International Workshop on Materials Science, which is incorporated by reference herein in its entirety. This discusses the use of reverse biasing to create avalanche breakdown and subsequent emission of visible light in pn-junctions.
Therefore, what is needed is a new radiation source that is useful in lithography, as well as, lithography apparatus and device manufacturing methods using the source.